How to Grow Polygonum Plants

Guide to Growing Knotweed, Bistort and Wiregrass

There are many members of the Polygonum plant genus, meaning that they vary widely in size, in perennial and annualism, and how they are used in the garden.

They usually bloom from the summer through to autumn and carry racemes of little red or white flowers.

Knotweed

The range in size and nature of Polygonum means that they can be used in many parts of the garden; however many members are aquatic and so can be used in boggy parts of the garden or in water features.

Vietnamese coriander

Some common names for Polygonum include Knotweed, Bistort, Russian Vine, Vietnamese Coriander, Smartweed and Prince's feather.

Commonly Grown Polygonum Species

Polygonum amphibium stipulaceum

Polygonum amphibium
Polygonum amphibium stipulaceum - Water Knotweed by Gmayfield10; creative commons.

Polygonum aviculare

Polygonum aviculare
Polygonum aviculare - Common Knotgrass by Matt Lavin; creative commons.

Polygonum multiflorum

Polygonum multiflorum
Polygonum multiflorum (Tuber fleeceflower / Chinese climbing knotweed / He shou wu), photograph by 阿橋 HQ; CC.

Polygonum cuspidatum

Polygonum cuspidatum
Polygonum cuspidatum (Japanese knotweed / Asian knotweed), picture by Matt Lavin; CC.

Polygonum Plant Growing and Care Guide

Common Names: Knotweed, Bistort, Wiregrass, Tear Thumb, Knotgrass, Arssmerte, Mile a Minute, Smokeweed, Swamp Smartweed.
Life Cycle: Half hardy annual, hardy annual. Half hardy perennial, hardy perennial.
Height: 2 to 135 inches (5 to 350 cm). Vines may grow up to 100 feet (30 M).
Growing Region: Annuals: zones 3 to 10. Perennials: 3 to 9.

Native: Temperate regions of Europe and Asia.
Flowers: Summer and autumn.
Flower Details: White, pink, green. Sometimes inconspicuous. Prominent sepals. Flowers on stalk.
Foliage: Oblong, Leathery.

Sow Outside: Cover seed. Following the last frost. Spacing 8 to 50 inches (20 to 130 cm).
Sow Inside: Germination time: three weeks to two months. Temperature 70°F (21°C). One month before expected last frost. Transplant outdoors following the last frost.

Requirements: Light shade; can grow in the sun in cold areas. Good drainage. Soil pH 6 to 7. Moist soil. Regular watering. Prune in spring to tidy the plant. Perennials should have their stems cut back in the autumn. Propagate: dividing in the spring in cool areas or in the autumn in warm areas.
Family: Polygonaceae.
Miscellaneous: Some members are considered to be invasive species in many areas.

How to Grow Polygonum Plants Such as Knotweed in the Garden

If growing outdoors from the off then it is best to sow Knotweed and other Polygonum species outside just after the last frost of spring; once sown lightly cover the Knotweed seeds with topsoil.

The plants prefer to grow in a partially shaded part of the garden and like to have a moist soil of pH 6 to 7.5.

Even though the soil should be moist it is important that Knotweed grows in areas of good drainage.

If starting Polygonum indoors then do so about three weeks before you expect the last frost.

The seeds can take anything from 3 to 8 weeks to germinate at a temperature of 21 to 24 degrees centigrade (70 to 75°F).

Once ready, transplant the seedlings at a spacing of 20 to 30 cm (8 to 12 inches) for small Polygonum species; 30 to 60 cm (12 to 24 inches) for medium size; or 70 cm to 1.2 m (30 to 48 inches) for larger Polygonum varieties.

Caring for Knotweed - Polygonum

Knotweed and other Polygonum plant genus members like to grow in a moist garden soil, so keep well watered.

The vine varieties of Polygonum should be pruned heavily at the start of spring to prevent scrawny growth and to make them more attractive.

Once flowering has finished, cut back the stems (usually in the autumn).

If you require more plants then they can be propagated by division in the autumn.

I hope that you enjoyed this guide on how to grow Polygonum plants. You may also enjoy the following Gardener's HQ growing guides: How to grow Sorrel and Rhubarb plants.